mysql> select
    -> unix_timestamp(),
    -> unix_timestamp(ts),
    -> round(unix_timestamp(ts)),
    -> max(unix_timestamp() - round(unix_timestamp(ts)))
    -> from heartbeat
    -> where server_id = 3;
+------------------+--------------------+---------------------------+---------------------------------------------------+
| unix_timestamp() | unix_timestamp(ts) | round(unix_timestamp(ts)) | max(unix_timestamp() - round(unix_timestamp(ts))) |
+------------------+--------------------+---------------------------+---------------------------------------------------+
|       1440604167 |         1440604168 |                1440604168 |                                                -1 |
+------------------+--------------------+---------------------------+---------------------------------------------------+
1 row in set (0.00 sec)